Method for producing spherical particles having a narrow size distribution

ABSTRACT

A method is provided for producing small diameter spherical particles in a narrow size distribution from low viscosity melts. Inert gas is constrained to uniformily envelope and move cocurrently with a free stream extruded from the melt. The stream attenuates in diameter and disintegrates into spherical-like droplets under the influence of surface tension. The droplets quickly solidify into small diameter spherical particles having a narrow size distribution.

[ 1 March 6, 1973 Unite States atent Rakestraw et a1.

2,892,215 6/1959 Naeser et al.............................264/9 FOREIGNPATENTS 0R APPLICATIONS METHOD FOR PRODUCING 712,699 7/1954 GreatBritain........................264/12 John W. Mottern, Cary, both ofN.C.

Primary ExaminerRobert F. White Assistant Examiner-J. R. Hall [73]Assignee: Monsanto Company, St. Louis, Mo. AttorneyVance A. Smith,Russell E. Weinkauf, Jonn h l E. W'll' 22 Filed: Dec. 3, 1970 D Up amand Nea [21] Appl. No.: 94,712

spherical particles in a narrow size distribution from low viscositymelts. Inert gas is constrained to uniformily envelope and moveco-currently with a free 349 HN 42 6. 21 01 B 1 M 6 M 2 6 9 2 l u 4 USCl Fieldoi Search....................

stream extruded from the melt. The stream attenuates into spherical-likedroplets under the influence of surface tension. The

[56] References Cited in diameter and disintegrates UNITED STATESPATENTS dropletsquickly solidify into smalldiameter spherical particleshaving a narrow size distribution. 3,556,780 1/1971 Holtz,

1122,494 6/1944 Ferguson ....264/12 4 Claims, 4 Drawing FiguresPATENTEDMAR 61975 SHEET 10F 2 INVENTORS LAWRENCE F. RAKESTRAW JOHNW.MOTTERN BY ATTORNEY PATENTED 61975 SHEET 2 CF 2 I I l I00 200 300 400500 600 700 800 900 v O O O 9 8 7 IOU-- O o O o 3 2 SPHERE DIAMETER,MICRONS FIG. 3.

u I I I00 200 300 400 500 600 700 800 900 SPHERE DIAM ETER, MI CRONSIOO- q 0 O 6 3 Amzomozz mmkmida wmwIam FIG. 4.

INVENTORS LAWRENCE F. RAKESTRAW JOHN W. MOTTERN ATTORNEY METHOD FORPRODUCING SPHERICAL PARTICLES HAVING A NARROW SIZE DISTRIBUTIONBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to the field of producing small diameter spherical particlesfrom low viscosity melts.

2. Discussion of the Prior Art There is a growing need for powders,particularly metallic powders, in a number of applications. For example,free-flowing spherical powders are necessary in hard surfacingoperations and also to facilitate automatic feeding in metal and ceramicspraying applications. There is a definite need for uniformily sizedspherical powders in flame and arc spraying.

Generally the materials comprising the particles are normally solid atroom temperature, i.e., about 25C, and in the molten state have very lowviscosities, usually below about poises. Such materials are the metals,metalloids, salts, intermetallic compounds, and the low viscosityelements, compounds, and compositions which may be made molten andextruded as free stream. Many techniques have been devised to producesmall particles from such materials. The techniques vary from pouring amolten stream of the material on a high speed rotating disk to atomizingthe stream with a jet of gas. Although small in size, the resultingparticles are generally not spherical and have a broad sizedistribution.

In still another technique, the material is extruded as a fine diametermolten stream. Since the molten stream has neglible viscosity, surfacetension quickly augments the inherent disturbances in the streamconfiguration, causing the stream to disintegrate into discrete dropletswhich solidify into sphere-like shapes. It has been noted that the sizedistribution of the spherical particles utilizing the free streamingtechnique is again undesirably broad.

Another obvious disadvantage of the latter technique is the need tochange the orifice plate each time particles having a different sizedistribution are required. When extremely small particles are needed,for example, particles with a diameter less than about 5 mils,manufacturing costs become high. The expense of producing orifice platesrapidly increases with diminishing orifice size.

As far as we have been able to ascertain, there are no prior artprocesses for the formation of small spherical particles with a largepercentage (by weight) of the particles falling within a 100 microndiameter range while simultaneously maintaining the essentiallyspherical shape. An important object of the present invention is toprovide an economical method by which the diameter of the particles maybe varied while maintaining the narrow size distribution.

SUMMARY When a molten stream of low viscosity is extruded through anorifice, a large variety of interactions between the stream and orificeare partially responsible for disturbances in the stream configuration.Friction between orifice wall and stream, minute changes in orificediameter, wetting characteristics of the molten material, and occlusionsin the molten material are all somewhat responsible for thedisturbances. The magnitude of the disturbances vary considerably suchthat when the stream disintegrates due to augmenting action of surfacetension, a population of spherical particles having a broad distributionof diameters are form ed upon solidification.

We have devised a method by which the distribution of disturbancemagnitudes occurring in the surface configuration of a free stream havebeen significantly narrowed. That is, the disturbances which result inthe configuration of a stream when practicing a method of our presentinvention are essentially the same. This results in the formation ofspherical particles having a narrow size distribution.

Briefly, in accordance with one aspect of the present invention, themolten material is extruded as a free stream. An essentiallycylindrical, co-current flow of inert gas unreactive with the moltenmaterial is provided to envelope the issuing stream uniformly about itscircumference. The inert gas generally has a velocity greater than thestream extrusion velocity but preferably less than the sonic velocity.By inert" gas, we mean a gas which is substantially unreactive with themolten stream. The sonic velocity of a gas varies according to itscomposition. Helium, for example, has a sonic velocity of about 98,500cm/sec. at standard temperatures and pressure.

For reasons not completely understood, the use of a co-current flow ofinert gas is believed to narrow the distribution range of disturbancemagnitudes and thus to allow surface tension to break-up the stream intodroplets of essentially the same size. The low velocity flow of theinert gas avoids turbulence such as noted in atomizing techniques whichutilize jets of gas significantly higher than sonic velocities. Thevelocity of the inert gas being greater than the stream extrusionvelocity not only promotes narrow size distribution of particles, butalso provides a stream attenuation greater than the attenuation whichnormally occurs due to the effect of gravity. This latter effect permitsthe use of a single orifice to obtain spherical particles of a desiredsize.

Other objects and advantages may be best understood from a reading ofthe following description and the accompanying drawings:

FIG. 1 is a vertical cross-section of an extrusion apparatus which maybe used in accordance with the present invention; and

FIG. 2 is an enlarged view of the orifice and gas plates of FIG. 1; V

FIGS. 3 and 4 are graphs depicting comparative screen analysis data.

DESCRIPTION It is known that when a liquid of low viscosity is extrudedas a free stream that the stream inherently has minute disturbances inits surface configuration. The disturbances are caused by many factorssuch as orifice geometry, occlusions within the stream, turbulence dueto stream interaction with surrounding gases, stream velocity profiledue to friction with the orifice material, wetting characteristics ofthe molten material and orifice, etc. Left unaffected, the disturbancesaugmented by the surface tension of the molten material, grow in sizeand propagate along the stream. In a very short time interval, less thana second, the stream disintegrates into myriads of droplets which tendto assume a spherical shape and solidify. Because the disturbances havea broad distribution of magnitudes, the spheres have a broaddistribution of sizes.

We have found that the present invention may be practiced on a varietyof materials such as metals, metalloids, metal oxides, intermetalliccompounds, salts and other normally solid materials having low viscositymelts. While we do not wish to limit ourselves to a particular material,in the interest of clarity the following description utilizes themetals, particularly copper, as working examples of normally solidmaterials with low viscosity melts.

FIG. 1 illustrates an apparatus for carrying out the method of theinvention which we have successfully employed in the formation ofspherical particles having a narrow size distribution. The apparatus iscomprised of three major sections: a cylindrically-shaped susceptor thecrucible assembly 11; and pedestal assembly 12. Pedestal assembly 12supports both susceptor 10 and crucible assembly 11. An inductor coil 13is helically wound about susceptor 10 and is coupled to an appropriatepower source (not shown). Alternatively, resistance heating may be used.

Crucible assembly 11 positioned within susceptor 10 is comprised of abase or orifice plate 14, cylindrical upright walls 15, and a removablegas-tight cap 16. Although not shown, cap 16 is generally provided withentrances for both an inert gas and material feed. Plate 14 contains anorifice l7. Pedestal assembly 12 not only acts as supporting means butalso provides gastight chamber 18 and inert gas channels 19. Chamber 18is also gas tight.

The composition of the various parts of the extrusion assembly dependslargely on the compatibility with the material to be extruded. Thus, forhigh temperature molten metals, it is necessary that high melting pointcompositions such as a ceramic by employed, particularly where the partscome in contact with the molten material. It is also essential thatparts be relatively inert in the presence of the molten material.

The general operation of the extrusion assembly is simple. A quantity ofmaterial is fed into crucible assembly l1. Inductor coils 13 provide anelectric field which causes susceptor 10 and, consequently, crucibleassembly 11 to heat up thereby forming melt 20. Gaseous pressure abovemelt 20 extrudes melt 20 through orifice 17 as a free stream 21 intochamber 18. The stream rapidly breaks up into droplets which, in thecase of normally solid materials, quickly solidifies into particles.

The above-described apparatus, however, does not by itself producespherical particles having a narrow size distribution. To accomplishthis, a plate 22 is positioned below orifice plate 14 with throat 23 ofplate 22 being substantially co-axially aligned with orifice 17. Apredetermined flow-rate of inert gas is introduced .through channels 19between plates 14 and 22. The gas is constrained to flow in a radiallyinward direction by adjacent plates 14 and 22 toward throat 23. This maybe seen best in the expanded view of plates 14 and 22 furnished by FIG.2. The gas then exits at throat 23, uniformily enveloping molten stream21 due to its radial flow and throat 23, forming a cylindrical shell 24(seen in dotted outline) about the stream, and then moving co-currentlywith the stream.

It is desirable that the uniform and cylindrical flow of the inert gasextend from the orifice exit to a point at which the stream breaks intodroplets due to surface tension. Stream breakup ordinarily occurs at adistance of 100 d to 200 d below the orifice where d is the diameter ofthe orifice. Constraint of the inert gas in cylindrical form may beaccomplished by providing plate 22 with a throat 23 which extends to adistance l00 d to 200 d below plate 14. Thus, longer throats are notneeded.

Atomization techniques use jets of gas from psi up to destroy the streamcontinuity. Such pressures results in velocities significantly higherthan sonic velocities. In contrast, gas velocities employed in thepresent invention are generally subsonic since we have noted that gasesat sonic velocities or greater have a deleterious effect upon streamcontinuity. Thus, for melt extrusion velocities of to 2,000 cm/sec. theinert gas velocity of helium, for example, may, at a temperature of1400C range from a value somewhat greater than the extrusion velocity tovelocity of around 220,000 cm/sec. It should be noted that thevelocities employed for optimum conditions will vary according to thetype of inert gas used and its temperature.

We have also noted that the thickness of the gas shell has an influenceon the uniformity of gas flow. Thus, to avoid gas turbulence, we havefound it desirable to limit the thickness of the gas shell to less thanabout 30 times the orifice diameter with the lower limit being about thesame as the orifice diameter. One way in which this may be accomplishedis by providing a throat which has a diameter 1 to 30 times the orificediameter.

To attain the results in the following Tables, an apparatus similar tothat of FIG. 1 was used. A charge of copper was made molten and thenextruded through a 5 mil orifice. The spinning and gas pressure werevaried as shown in the Tables. The plate throat diameter was about 20mils and aligned coaxially with the orifice. The stream disintegratedinto droplets and solidified into a quantity of spherical particles. Ascreen analysis was conducted on each group of particles to determinethe percent (by weight) of particles in each size range. For clarity,the mesh size of the screen was converted into microns. Table 1represents a neglible flow of inert gas compared to the sonic velocityof helium at 1,400C of 220,000 cm/sec. For heavier gases such as argonor nitrogen, the velocity required is considerably less.

TABLE l Spinning Pressure 30 psig. lnert gas pressure 05 psig. Extrusionvelocity 706 cm/sec Inert gas velocity 44,000 cm/sec Particle Size(Microns) Percent (by weight) under 355 2 355-425 l2 425-500 43 500-60043 TABLE 2 Spinning Pressure 30 psig. Inert gas pressure 8.0 psig.Extrusion velocity 6l0 cm/sec lnert gas velocity 176,000 cm/sec ParticleSize (Mlcrons) Percent (by Weight) 155-205 28 205-300 54 300-425 15425-500 3 TABLE 3 Spinning Pressure 30 psig. Gas pressure 12 psig.Extrusion velocity 552 cm/sec Gas velocity Particle Size (Mierons)Percent (by weight) It is important to note the distinctions between theresults obtained. For example, in Table l, the majority of particles arebetween 425 to 600 microns, a range of 175 microns. In table 2, overhalf of the particles are between 200-300 microns in size. A dramaticpeaking of the distribution may be seen in Table 3 where 65 percent ofall particles are between 155-205 microns and about 94 percent between155-250 microns. Examination of the particles revealed that the shapeswere essentially spherical.

To better illustrate these comparative results of Ta bles l, 2, and 3,reference is now made to the graph of FIG. 3. Curves 30, 31, and 32 wereconstructed by using as coordinates the midpoint of each size range asattained from the screen analysis and the corresponding percentage ofparticles within the particular size range. With increasing inert gaspressure below the orilice and therefore velocity, there is both asignificant shift toward smaller particle size but also a pronouncednarrowing and peaking of the distribution of particles. For example,curve 30 depicts a size distribution from about 300 to 600 microns witha peak percentage appearing in the 500 microns range. On the other hand,curves 31 and 32 depict smaller distributions with higher peaks. Thus,the effect of the co-current flow of inert gas is significant both innarrowing the distribution size and shifting the peak distribution.

Tables 4-6 tabulate the screening analysis results for a higher spinningpressure (and high extrusion velocity) for varying inert gas pressures.

TABLE 4 Spinning Pressure 40 psig. Inert gas pressure 2.5 psig.Extrustion velocity 796 cm/sec Inert gas velocity 155,000 cm/secParticle Size (Microns) Percent (by weight) 355-420 10 420-500 34500-595 45 595-700 I 1 TABLE 5 Spinning Pressure 40 psig. Inert gaspressure 8.3 psig. Extrusion velocity 732 cm/sec Inert gas velocity179,000 cm/sec Particle Size (Microns) Percent (by weight) 150-205 25205-250 26 51 250-295 24 295-355 215,000 cm/sec 1O TABLE 6 SpinningPressure 40 psig. Inert gas pressure 12 psig. Extrusion velocity 810cm/sec Inert gas velocity 215,000 cm/sec Particle Size (microns) Percent(by weight) Table 6 shows that 92 percent of the particles formed inthis run fell between 150-250 microns. This should be contrasted to theresults obtained for Table 4 whereupon 45% were obtained between about500-595 microns. FIG. 4 depicts a family of curves 40, 41, 42 generatedfrom the data in Tables 4, 5, and 6, respectively, and similar to thoseof FIG. 3. Again, due to the increase in inert gas velocity, there is apronounced shifting to smaller particle sizes, and narrowing and peakingof the particle distribution. Examination of the particles revealed thatthey had essentially spherical shapes.

In summary, it may now be seen that we have been able to fabricatespherical particles which are within a narrow size range through thepractice of a novel method in accordance with the present invention.Inert gas is constrained to flow co-currently in cylindrical form aboutthe stream while maintaining the velocity of the gas greater than thestream velocity but less than a velocity which would substantiallyeffect the stream due to aerodynamic phenomena. Preferably, the velocityof the inert gas is kept below its sonic velocity. The cylindrical shapeis preferably maintained until the stream has broken up into particlesdue to the augmenting effect of surface tension. This distance rangesfrom about to 200 times the diameter of the extrusion orifice.

As stated hereinbefore, it is cumbersome and expensive to continuallychange the orifice plate when different size spherical particles areneeded. Similarly, it is expensive to fabricate small particles since itis necessary to use orifices of small diameter. In accordance with thepresent invention, attenuation of the stream diameter may be closelycontrolled through predetermined variations in the velocity of the inertgas. Thus, the peak size distribution of the spherical particles mayalso be varied as desired as illustrated by FIGS. 3 and 4 withoutchanging the size of the orifice.

Although copper was employed to demonstrate the formation of particles,it is to be understood that other normally solid materials with lowviscosity melts such as those named herein may be used when practicingthe present invention. It is to be understood that alterations, changes,and modifications may be made by those skilled in the art after areading of the descriptive matter contained herein yet fall within thescope of the invention as defined by the following claims:

What is claimed is:

l. A method for forming spherical particles from a normally solidmaterial having a viscosity of less than about 10 poises in the moltenphase comprising a. extruding said material in the molten phase throughan orifice as a free stream;

velocity is less than its sonic velocity.

3. The method of claim 1 in which the thickness of the inert gasenvelope is maintained between 1 and 30 times the orifice diameter.

4. The method of claim 1 in which the cylindrical flow of inert gas ismaintained from the orifice to a point therebelow a distance to 200times the orifice diameter.

1. A method for forming spherical particles from a normally solidmaterial having a viscosity of less than about 10 poises in the moltenphase comprising a. extruding said material in the molten phase throughan orifice as a free stream; b. enveloping the said stream with acylindrically shaped co-current flow of inert gas immediately uponemergence of the said stream from the said orifice such that velocity ofthe said inert gas is greater than the extrusion velocity of the saidstream causing the said stream to attenuate, c. allowing the stream todisintegrate into droplets due to the effect of surface tension, and d.freeze as solid spherically shaped particles.
 2. The method of claim 1in which the inert gas velocity is less than its sonic velocity.
 3. Themethod of claim 1 in which the thickness of the inert gas envelope ismaintained between 1 and 30 times the orifice diameter.